1. Field of the Invention
The present invention related to a behavior control of a vehicles such as an automobile, and more particularly, to an improvement of such a behavior control device with regard to the reliability of its normal operation periled by a failure of a longitudinal acceleration sensor incorporated therein.
2. Description of the Prior Art
A wide variety of behavior control devices have been developed in the field of the modern so-called vehicle stability control, the art of controlling a running behavior of a vehicle such as an automobile against such instabilities as a spin, a drift out and others, the control being based upon dynamic control calculations made instantly by an electronic computer incorporating various operation parameters of the vehicle, such as longitudinal and lateral accelerations, vehicle speed, yaw rate, steeling angle, engine operation parameters, etc.
Those various prior art behavior control devices have their particular advantageous operational features. However, it will go without saying that those advantageous operational features will be actually exhibited only when the sensors for detecting various operation parameters of the vehicle needed for the control calculations are operating normally.
In various sensors for detecting the operation parameters of the vehicle, an acceleration sensor for detecting the longitudinal or lateral acceleration of the vehicle body is one of the most important for ensuring a high quality of the vehicle stability control. Nevertheless, the acceleration sensor is constructionally very delicate, and is relatively liable to a malfunction. At present, most of the acceleration sensors for the automobile applications have such a fundamental construction that an inertial mass element movable along a linear guide is biased by a pair of springs from linearly opposite sides thereof so as to be positioned at its neutral position in the absence of any acceleration.
The acceleration sensors of such a fundamental construction will operate unchangeably over a long period, if they are subjected to accelerations acting only along the linear directions. However, in the automobile applications, the acceleration sensors are always subjected to accelerations acting in various directions, regardless whether they are used as a longitudinal acceleration sensor or a lateral acceleration sensor, such that the inertial mass element is often rubbed against the linear guide while it is shifted therealong. Therefore, the acceleration sensors often unavoidably cause a gradual or an abrupt error in their outputs due to a sticking of the inertial mass element to the linear guide. How often and how strong the inertial mass element is rubbed against the linear guide depends on the driver of the vehicle and his/her principal driving area. Therefore, it is very difficult to estimate how long the normal operation of the acceleration sensor is guaranteed.
In Japanese Patent Laid-open Publication 7-196029 (1995), there is proposed a device for judging a failure of a longitudinal acceleration sensor of a vehicle, comprising means for calculating rotation speed of a wheel based upon an output of a wheel speed sensor, means for calculating an estimated vehicle body acceleration based upon the wheel rotation speed, a longitudinal acceleration sensor for detecting longitudinal acceleration of the vehicle, means for calculating a ratio of the vehicle longitudinal acceleration detected by the longitudinal acceleration sensor to the estimated vehicle body acceleration together with an integration calculation of a first integrated value which is decreased when the ratio is within a predetermined range while it is increased when the ratio is outside of the predetermined range, and means for conducting a predetermined failsafe processing when the first integrated value is larger than a predetermined value.
However, the difficulty in judging the normal operation of the acceleration sensors of the vehicles resides in that the vehicles do not always run on a horizontal road, occasionally climbing an uphill road or descending a downslope road, these inclinations all affecting the ostensible outputs of the acceleration sensors having the above-mentioned fundamental construction.